add -p: prompt for single characters

add -p: prompt for single characters

Use Term::ReadKey, if available and enabled with interactive.singlekey,
to let the user answer add -p's prompts by pressing a single key.  We're
not doing the same in the main 'add -i' interface because file selection
etc. may expect several characters.

Two commands take an argument: 'g' can easily cope since it'll just
offer a choice of chunks.  '/' now (unconditionally, even without
readkey) offers a chance to enter a regex if none was given.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>